Argh! Mystery nitrite!

It's possible you could be getting a false-positive reading from your nitrite test. Did you add dechlorinator/chemicals before you tested? These chemicals are known for causing false-positive readings. Are you using strip tests, or the liquid ones? Strip tests can be inaccurate..

Also, sometimes the water company will throw a bit more ammonia at you than normal, it's happened to me a couple of times. Think of it like a free catalyst for your new tank cycle, lol. Putting live plants in will help.

your test kit could be off, paper dip strips are extreamly inaccurate. the water source could be really off, check yours and a family or friends to see the difference.

The sand or the water????

test kits normally have a life of about 6 months once opened

I had this issue a while back and I know of one other person who did too, turned out, in both cases it was down to decor items......we both had those neon soft coral type plants in, the rubbery, silicone ones, for some reason some of them seem to give off traces of nitrite, maybe not noticeable in a mature tank but a cycling or newly cycled tank can't seem to cope.

Not saying this is the case with you, but just thought Id check, as my issue and this other persons was solved the moment we removed the plants.......very strange but I tested it a few times over the course of weeks and I always got nitrite readings within a day or so of adding the plant and it was gone when removed, every time. I thought it was fluke till this other person found the same thing.

Thanks for all your suggestions, but it's still not helping. The sand is new and the same as I use in another tank. It's not the test kit as the other tanks read 0. (drop test) One ornament which was previously used in another tank. Haven't added anything to the water for days.

Maybe it's a duff filter?

Hmmm...that is really odd, have you tried putting some more mature filter media in from another tank just to test or perhaps swapping the filters around????